High performance Micro-Electro-Mechanical Systems (MEMS) inertial instruments (accelerometers and gyros) require closed-loop operation. Good performance under vibration requires that the magnitude of the electrostatic feedback force be highly insensitive to the position of the instrument's inertial mass relative to it's null position. Because of their high force dependency on position, the typical electrostatic vertical comb drive as employed in MEMS actuators cannot be applied to precision MEMS inertial instruments.
FIG. 1 illustrates an instrument that performs acceptably under static accelerations. However, if vibration were superimposed on a static acceleration, a large error in the time-average measured acceleration would result. This error is called vibration rectification, whose nature and sources are well known by those in the field. One major source of vibration rectification error in closed-loop instruments is the proof mass positional-dependence of the feedback force. When the rotor and stator teeth are substantially aligned (same height), the electrostatic force between the rotors and the upper or lower half of the stators changes greatly with a small vertical movement of the rotor teeth. As these rotor teeth move up and down with applied external vibration, the time average differential voltage that results is different from the voltage that would occur with only the static acceleration input. This is an accelerometer error. The 2nd order portion of the force-position dependency is the main source of the error, but 1st and higher than 2nd order components can also contribute. The contribution to the error from the various order components depends on the details of the design. Regardless, the need is for a design which significantly reduces this position dependency, especially a 2nd order dependency.
Therefore, there exists a need for an electrostatic MEMS drive, which has very low position sensitivity.